1. Field of Invention
The invention relates to a boiler for combustion of solid fuel, especially biomass, with a boiler wall and a boiler cover, with one combustion site as the first combustion stage, with a primary air supply means, with an ignition means, with a supply means for conveyance of fuel to the combustion site, with a burn-up and calcination space as the second combustion stage, with a combustion chamber which is open to the top and with a flue, the burn-up and calcination space being located underneath the combustion site such that fuel can be moved from the combustion site into the burn-up and calcination space and further burns up or calcines there, the combustion site and the burn-up and calcination space being located jointly in the combustion chamber, and the flue adjoining the combustion chamber to the top so that the flue gases which are formed in the burn-up and calcination space burn up together with the flue gases which form over the combustion site in the flue.
In addition, the invention relates to a process for producing heat energy by burning a fuel, especially biomass, in a boiler, the fuel being burned in a first combustion stage and the fuel which has been burned up or partially burned up in the first combustion stage and which still has a portion of carbon, burning up or calcining further in the second combustion and calcination stage.
2. Description of Related Art
To produce heat energy, generally, combustible substances are burned in order to use the thermal energy obtained in doing so for heating of media. Heating takes place using a heat exchanger, for example an air-water heat exchanger, in which water is heated by the hot air which is formed when the fuels are burned. In addition to classical boilers, in which fossil fuels, such as for example petroleum, natural gas or coal are burned, there are also boilers in which renewable raw materials, especially wood in the form of chips and pellets, are used as fuels. These pellet boilers which are also called piece wood boilers are in the meantime available in a very large output range from roughly 5 to 100 kW.
Within the framework of this invention, the term “biomass” is defined as renewable raw materials. It includes not only wood, especially in the form of wood shavings, wood chips or wood pellets, but also grains as well as grain-like materials, such as rape or straw, the latter preferably in the form of rape press cake or straw pellets.
In the operation of a boiler which is designed for wood pellets with grain, a series of problems has arisen so that, in the past, operation with the same quality as with the proper fuel could hardly be achieved. The major problems here lie in poorer efficiency due to the poorer burn-up of the grain, increased emission of dirt, carbon monoxide, hydrocarbon and nitrogen oxides, by which the allowable boundary values of the boilers are often exceeded, and an increased ash content which leads to problems in ash removal and to problems from slagging.
A boiler of the type underlying the present invention is known from European Patent Application EP 1 288 570 A2. For the known boiler, in addition to the actual combustion site—the first combustion stage—there is a burn-up and calcination space as the second combustion stage in which the fuels which are only partially burned up in the first combustion stage and which still have a portion of carbon and thus still some energy value, can burn up further. Since the burn-up and calcination space is located underneath the combustion site, the fuel can easily travel from the first combustion stage to the second combustion stage by the fuel falling from the combustion site into the burn-up and calcination space.
Encapsulation of the first and second combustion stage within a common combustion chamber results in that the flue gases which form when the fuels burn up or calcine in the burn-up and calcination space and which have poorer exhaust gas values as a result of the somewhat lower temperatures prevailing there, are supplied to the flue and combustion gases which form when the fuel is burned on the first combustion stage, and burn up jointly with them within the flue in the high heat prevailing there, so that the exhaust gas values of the boiler are hardly adversely affected by the “poor” exhaust gas values from the second combustion stage.
In the known boiler, thus, not only is the efficiency increased by the energy value of the fuels being almost completely used up, but in addition the actual associated “poorer” flue gases are for the most part neutralized by these flue gases being routed through the extremely hot first combustion stage and burning further there.
In spite of all the advantages which are enjoyed by the known boiler, there are problems in adherence to future, even stricter emission values, especially when biogenic fuels which are very protein-rich are to be used.